Transimpedance circuits are known. In its basic form, such a circuit is simply a current-to-voltage converter consisting of a current transformer with a resistive shunt connected across its secondary terminals, a voltage proportional to, and in phase with, the primary current appearing across such shunt. This voltage can then be amplified to provide a rated output voltage, say in the range of 1 V to 10 V. Alternatively, instead of a shunt, an operational amplifier with a resistive feedback, followed by a gain amplifier, can be used. Such transimpedance circuits have been used as current transducers in high-voltage loss measuring systems. See, for example, the circuits illustrated in "Calibration of Test Systems for Measuring Power Losses of Transformers" by S.P. Mehta et al., published in IEEE Transactions on Power Delivery, Vol. PWRD-1, No. 4, October 1986.
However, for such a circuit to provide accurate results, adjustments have to be provided to correct for the in-phase and quadrature errors at each gain setting of the amplifier.
The output of a transimpedance circuit is given by EQU E.sub.o =(I.sub.p /n).R.G.[1+(.alpha.+j.beta.)] (1)
where .alpha. and .beta., respectively, are the in-phase and quadrature errors of the circuit, G is the gain of the output amplifier, R is the shunt resistance, I.sub.p is the primary current, and n is the nominal ratio of the current transformer. During calibration, .alpha. and .beta. can be adjusted to zero. However, due to temperature effects and changes (instability) in the accuracy adjustments, it is difficult to maintain a magnitude and phase accuracy of better than .+-.100 ppm and .+-.100 .mu.rad, respectively, at all gain settings.